Glioblastoma (GBM), a uniformly lethal brain cancer, is characterized by diffuse invasion and abnormal activation of multiple receptor tyrosine kinase (RTK) signaling pathways (1). Despite current therapies, the prognosis for GBM is poor and mean survival remains less than 2 years. An improved understanding of the mechanisms driving abnormal cell signaling is essential for improving treatment outcomes. The long-term goal of this innovative proposal is to define tumor-microenvironment interactions critical in brain cancer and identify clinically relevant, druggable therapeutic targets. Specifically, we focus on the role of extracellular heparan sulfate proteoglycans (HSPGs) as they regulate the activity of multiple ligand-mediated signaling pathways (2), are altered in malignant brain tumors (3, 4), and have the potential to influence both tumor cells and critical tumor-microenvironment interactions, including the tumor-associated microglia/macrophage response. HSPGs, present on the cell surface and in the extracellular matrix, regulate signaling via their ability to bind and alter the bioavailability of diverse ligands, including growth factors, morphogens, chemokines, and enzymes. SULF2, an extracellular heparan sulfate endosulfatase, actively regulates HSPG-dependent signaling by removing the sulfate from 6-O- of glucosamine (6OS) and liberating protein ligands from HSPG sequestration (5). Alterations in HSPG core protein expression and SULF2 expression are common in diverse cancers and the PI of this proposal has shown SULF2 can drive carcinogenesis in malignant astrocytoma through regulation of RTK signaling pathways. As extracellular enzymes that are both tethered to the cell membrane and secreted, the SULFs and their HSPG substrates are present in the extracellular environment and have great potential as novel therapeutic targets. Our Aims are: Aim 1: In human infiltrating astrocytomas, identify the alterations in HSPG expression and sulfation associated with tumor malignancy. Aim 2: Determine HSPG changes driving tumor biologic behavior, including microglia/macrophage response to tumor. Aim 3: Identify how HSPG alterations activate signaling pathways to promote GBM malignant behaviors. The proposed research will determine the mechanisms by which alterations in HSPGs drive oncogenic cell signaling pathways in malignant brain cancer and validate HSPGs as clinically relevant, novel therapeutic targets. Successful completion of these studies provides a preclinical basis to study agents that target HSPGs as a novel therapeutic option in malignant brain cancer.